Random access memory, or RAM as it is known in the art, provides short-term storage for digital electronic data. Most forms of RAM known in the art are volatile memory and, as such, require effectively constant application of power to maintain the information contained within the memory. Volatile memory stands in contrast with non-volatile memory, such as read-only memory, or ROM, which does not require constant application of power in order to maintain the data stored within the memory.
Static random access memory, or SRAM, is a particular type of RAM and is well known in the art. As with many common forms of RAM, SRAM utilizes arrangements of semiconductors to store digital information. While SRAM requires an essentially constant power source in order to maintain the digital information, SRAM contrasts with dynamic random access memory, or DRAM, in that SRAM effectively maintains data in the SRAM for as long as power is maintained to the SRAM and does not require stored digital data to be periodically refreshed.
While SRAM is an effective and cost-effective digital memory, the need to supply power to the SRAM makes the SRAM costly to maintain, particularly with respect to power consumption. In general, a voltage approximately equivalent to the threshold voltage of the semiconductor transistors utilized in the SRAM needs to be maintained to the SRAM to maintain the integrity of the data. Because the electrical characteristics of a SRAM array, in particular semiconductors, and in various cases transistors, may tend to vary slightly owing to variations in manufacturing processes, the voltage requirements to maintain data stored in the SRAM may vary among different SRAM cells.
Furthermore, the threshold voltage of the semiconductors may vary depending on the temperature of the semiconductor. In certain circumstances, as temperature of transistors of an SRAM array increases, leakage current of the transistors of the SRAM array may increase, resulting in a decrease in a minimum supply voltage which may maintain data in the SRAM array. Conversely, reductions in transistor temperature may lessen leakage current and increase minimum supply voltages which may retain data stored in the SRAM array.
As a result, under certain circumstances, the voltage, and consequently the power consumption, required to reliably maintain the data in the SRAM array may be higher than what is required by most of the SRAM cells in the SRAM array. The voltage applied to the SRAM array to provide reliability may be, in various embodiments, the voltage which is needed to maintain the data in the one SRAM cell in the SRAM array which has the highest voltage threshold. In applications such as implantable medical devices, where increased power consumption may result in decreased longevity for the implantable medical device and increased requirements for surgical procedures, such increased power consumption may be highly undesirable.
U.S. Pat. No. 7,684,262, Zampaglione et al, discloses an SRAM leakage reduction circuit. A circuit can maintain a virtual ground node at a virtual ground reference voltage of Vdd−(1.5*Vth), or maintain 1.5*Vth across the memory cells, where Vth is a threshold voltage of an SRAM memory cell transistor and Vdd is a positive supply voltage. By tracking the Vth of the memory cell transistors in the SRAM array, the circuit reduces leakage current while maintaining data integrity. The circuitry controls the virtual ground node VG based on a memory transistor threshold voltage in order to safely keep the memory cell data under all process conditions.